Immunopharmacokinetic studies showed that four of seven biological response modifiers (BRMs) tested had a potent ability to augment natural killer (NK) cell tumor cytotoxicity in vivo, (MVE-2, Poly ICLC, Picibanil and AlphaBeta IFM). The same four BRMs also strongly stimulated MPhi tumoricidal activity, which remained elevated for over 10 days, in contrast to 7 days for NK cell activity. Multiple treatments with the 4 BRMs did not maintain elevated NK activity but resulted in decreased activity, in contrast to a maintained MPhiactivity. Such hyporesponsiveness to NK boosting by multiple treatments with BRMs was due to a decrease in large granular lymphocyte (LGLs), which are associated with NK cell activity, indicating a failure to maintain the expansion of LGLs. Seven BRMs were examined in vitro and in vivo for their capacity to induce the production and secretion of regulatory factors (colony stimulating factor, CSF: Prostaglandin E1 and E2, PGE; Interferon, IFN). Poly ICLC, Picibanil, AlphaBeta IFM and BM 41.332 stimulated MPhi to secrete significant amounts of CSF and PGE. Poly ICLC also stimulated IFN secretion. In vivo, Poly ICLC and MVE-2 treatment resulted in significant elevation in serum of CSF but not of PGE. The increased CSF was found to correlate with increased bone marrow (BM) cells and stem cells (GM-CFU-C) developing from BM cells. Since cytoreductive chemotherapy of tumors also leads to depressed NK cell and BM cellularity, MVE-2 and Poly ICLC were examined for their capacity to restore both cell populations following cyclophosphamide (Cy) treatment. Both BRMs caused an earlier restoration of NK and BM cells. Studies to establish more effective antitumor treatment modalities showed that combined cytoreductive chemotherapy and BRM (MVE-2 or Poly ICLC) resulted in extended survival periods and a substantial number of long-term survivors. Timing of administering the BRM in relation to the cytoreductive agent was critical with a need to give the BRM within 34 days following chemotherapy.